Add metrics for supervised user whitelist installation and uninstallation, and absolute count.

tools/metrics/histograms/histograms.xml

Index: tools/metrics/histograms/histograms.xml
diff --git a/tools/metrics/histograms/histograms.xml b/tools/metrics/histograms/histograms.xml
index 2a69aea4b8894ef56af4a60a01dad5f4cbbe737b..de27ab3a5af7f5468013ae9e8ba2b7548b0779ae 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -12244,6 +12244,13 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="ManagedUsers.Whitelist.Count" units="whitelists">

[Ilya Sherman, 2015/02/06 23:45:47]

Optional nit: Entirely up to you, but I think ".Size" sounds slightly better to
me than ".Count".

[Bernhard Bauer, 2015/02/08 14:05:33]

Hm, I would interpret "size" as the size of a whitelist, not as the size of the
set of whitelists.

+  <owner>bauerb@chromium.org</owner>
+  <owner>treib@chromium.org</owner>
+  <owner>pam@chromium.org</owner>
+  <summary>The number of whitelists installed for a supervised user.</summary>

[Ilya Sherman, 2015/02/06 23:45:48]

Please document when this is recorded.

[Bernhard Bauer, 2015/02/08 14:05:33]

Done.

+</histogram>
+
 <histogram name="ManagedUsers.Whitelist.JsonParseDuration" units="milliseconds">
   <owner>bauerb@chromium.org</owner>
   <owner>treib@chromium.org</owner>